In order to inhibit the metal corrosion process or to prevent it as completely as possible, metal surfaces are conventionally coated with primers or lacquers which contain corrosion inhibitors as the active components. It has been known for a long time to use such compounds as zinc potassium chromate, zinc tetrahydroxichromate, strontium chromate, barium chromate, or red lead oxide as corrosion inhibitors in corrosion protective coatings. Recently, however, the attention of the manufacturers of such coatings has been directed to the environmental behavior of such corrosion inhibitors. As a result, such time-tried inhibitors as zinc potassium chromate have been increasingly replaced by more environmentally acceptable corrosion inhibitors such as zinc phosphate.
The corrosion inhibiting property of zinc phosphate is based on the formation of protective layers in anionic pH ranges. The protective electrochemical reactions of zinc potassium chromate, do not take place with zinc phosphate, so that, until now, a combination of zinc phosphate with at least one electrochemically active corrosion protective pigment has always been recommended (Deutsche Farben Zeitschrift (Defazet) 29 (1975) 13-17).
Such electrochemically active corrosion inhibiting compounds are, for example, zinc and/or lead salts of 3-nitrophthalic acid, 4-nitrophthalic acid, 5-nitroisophthalic acid, or mononitroterephthalic acid, as are described in the German published patent applications DE-AS 22 04 985 and DE-AS 25 02 781. While such substances show an improved corrosion inhibiting effect (corrosion protection value) as compared with the conventional inhibitors, it appears that, when applied in the usual binder formulations, the coating materials such as paints and lacquers containing such corrosion inhibitors exhibit poor stability with respect to their sedimentation and the formation of solid agglomerated sediments which considerably complicate the practical application of such coating materials.